# CS4040/5040 Homework # 6

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CS4040/5040
Homework # 6
(CS4040 75 pts / CS5040 85 pts)
(plus a possible 10point bonus)
1. (10 pts.) Coin Changing. Give a set of coins of size at least 5 for which the greedy
algorithm does not produce an optimal solution. Provide the optimal solution for this
set, showing all work for a dynamic programming solution. (i.e. show the required
table(s)). Your set of coins must include a 1 cent coin.
2. (10 pts.) Huffman encoding. Given that frequencies of letters to encode via Huffman
encoding are given by the following recurrence relation: f(n) = f(n − 1) + f(n − 2) +
2f(n − 3), and that the initial three letters (a, b, and c) have initial frequencies 1, 1,
and 2, what is the code for the first 8 letters? (i.e. a:1, b:1, c:2, d:5, e:9, f:18, g:37,
h:73). Ties should be broken by keeping the deeper tree on the left. If two trees have
equal depth, keep the one with the earliest letter in the alphabet on the left. Can this
solution be generalized to the first n letters? If so, provide the generalization.
3. (10 pts.) Given n points in the plane, develop an algorithm that can find all line
segments that contain 4 or more of the points in O(n
2
log n) time.
4. (10 pts.) Give a formal definition for the problem of finding the longest simple cycle in
an undirected graph. Give the formal definition of the related decision problem. Give
the language corresponding to the decision problem.
5. (10 pts.) Provide a dynamic programming solution (psuedocode) to the 0−1 knapsack
problem. Your algorithm should run in O(nP) time where n is the number of items
and P is the maximum weight in pounds that the knapsack can hold.
6. (15 pts.)
(a) State the formal decision version of the 0 − 1 knapsack problem.
(b) Describe how the solution to problem 5 could be used to solve the decision version
of the 0 − 1 knapsack problem. What is the time complexity of this algorithm
that solves the decision problem?
(c) Is the dynamic programming solution to the 0−1 knapsack problem a polynomial
time algorithm? You should assume that we are talking about the decision version
7. (10 pts.) Consider the language GRAPH-ISOMORPHISM = {hG1, G2i : G1 and G2
are isomorphic graphs}. Prove that GRAPH-ISOMORPHISM ∈ NP by describing a
polynomial-time algorithm that verifies the language.
1
November 29, 2021
8. (10 pts.) This problem is for extra credit. Given an m × n matrix A composed of all
integers and an m-vector c also composed of all integer values, consider the problem
of determining whether there exists an n-vector y whose entries are either 1 or 0 and
whose product with A is less than c, i.e. Ay ≤ c. This notation means that each entry
of the vector on the left is ≤ to each corresponding value of the vector on the right.
Prove or disprove that this problem is in NP.
CS5040 only:
9. (10 pts.) Exercise 34.2-9, page 1066.
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